As additional information regarding cell-to-cell recognitions among oral bacteria is discovered, it is becoming increasingly clear that a dynamic but organized microbial community exists in the oral cavity. The part of this complex community studied in this laboratory pertains especially to microbial ecology. Both the actinomyces and streptococci, which are thought to be early colonizers of the tooth surface, and the gram-negative bacteria, capnocytophaga and bacteroides, are being studied. First, coaggregation-defective mutants were isolated and antisera to the respective parent strains were prepared. The antisera were absorbed with whole cells of mutants and the adsorbed antisera were used to detect surface proteins in the parent that were missing in the mutants. Monoclonal antibodies to the proteins of Bacteroides loescheii PK1295 were prepared and subsequently used to indicate that a 75kD and 46kD protein were involved in mediating coaggregation with Streptococcus sanguis and Actinomyces israelii. Five bacteriophage from sewage and eight phage from human dental plaque, all of which infect A. viscosus, were compared. The five sewage phage and several human oral phage were indistinguishable morphologically. Two other human oral phage were much larger and very different by restriction endonuclease analysis of their respective DNA molecules. Coaggregation between A. naeslundii PK606 and various streptococcal reagent strains that represent the six streptococcal coaggregation groups has been used as a model to study the relationship of these coaggregation mechanisms to those observed between other oral bacteria and these streptococcal reagent strains. Coaggregation-defective mutants of PK606 exhibit properties that are consistent with the model that four basic kinds of surface structures mediate coaggregation between the actinomyces and streptococci. The results of each of these investigative approaches are focused on understanding the relationship of cell surface recognitions among oral bacteria and their role in microbial ecology.